With a compact disk (CD) or a digital versatile disk (DVD), it is common that recording of binary signals and detection of tracking signals are carried out by utilizing a change in reflectivity caused by interference of reflected lights from the mirror surface and the bottom of pits.
In recent years, phase-change type rewritable compact disks (CD-RW, CD-Rewritable) or phase-change type rewritable DVD (tradename: DVD-RW, DVD+RW, in this specification, rewritable DVD may sometimes be referred to as RW-DVD) have been used as rewritable optical recording media.
Such phase-change type CD-RW or RW-DVD utilizes a phase difference and a difference in reflectivity caused by a difference in the refractive index between an amorphous state and a crystalline state to detect recording information signals. A usual phase-change type CD-RW or RW-DVD has a structure comprising a substrate, and a lower protective layer, a phase-change type recording layer, an upper protective layer and a reflective layer, formed on the substrate, so that multiple interference of these layers can be utilized to control the difference in reflectivity and the phase difference and to provide interchangeability with CD or DVD. Further, recording on CD-RW or RW-DVD means recording by overwriting wherein recording and erasing are carried out simultaneously.
In the above recording by overwriting, the crystalline state is used as an unrecorded or erased state, and the amorphous state locally formed is used as record marks.
If the recording layer is locally heated to at least the melting point by a recording laser beam and then immediately quenched, amorphous marks will be formed irrespective of the state (crystalline or amorphous) of the recording layer before recording. Quenching is usually accomplished by instantaneously turning off the recording laser beam to dissipate the heat. On the other hand, if the recording layer is heated to a temperature of at least the crystalline temperature and lower than the melting point by a recording laser beam having a power weaker than during the recording, it becomes an erased crystalline state irrespective of the state (crystalline or amorphous) of the recording layer before recording. As described above, with a rewritable phase-change type medium, the heating and cooling process in the recording layer is controlled by a change in the power of the recording laser beam and in its intensity, to accomplish the overwriting. It is common that such a change in the intensity of the recording laser beam is carried out in a short time of not more than a few tens nsec.
Whereas, one of problems in using CD-RW or RW-DVD is that the recording velocity and the transfer rate are low.
The reference velocity (hereinafter referred to also as 1-time velocity) in recording/retrieving of CD is a linear velocity (in this specification, “a linear velocity” may simply be referred to as “linear speed”) of from 1.2 to 1.4 m/s. However, for CD-ROM, a high velocity retrieving at a level of 40-times velocity at the maximum has been already realized, and a low velocity at a level of 1-time velocity is used only for retrieving of musics or images. Usually, in up to 16-times velocity retrieving, a constant linear velocity mode (CLV) inherent to CD is used, but in 24 to 40-times velocity retrieving, the transfer rate, access and seek times for the outer periphery data have been remarkably speeded up by an application of a constant angular velocity mode partly at the inner peripheral portion (P-CAV).
As a peripheral memory device for a computer, CD-RW has already accomplished 32-times velocity at the maximum by the P-CAV mode. On the other hand, write-once type CD-R has already accomplished 52-times velocity recording at the maximum and also for CD-RW, it is desired to further increase the transfer rate in recording.
On the other hand, the reference velocity (hereinafter referred to also as 1-time velocity) in retrieving of DVD is a linear velocity of 3.49 m/s, but with DVD-ROM, high velocity retrieving at a level of 16-times velocity at the maximum has already been realized, and a low velocity at a level of 1-time velocity is used only for retrieving of musics or images.
Speeding up in recording is in progress also for RW-DVD, but in the CLV mode, it is still at a level of 4-times velocity at best. On the other hand, again write-once type RW-DVD has accomplished 8-times velocity recording at the maximum, and also for RW-DVD, it is desired to further improve the transfer rate in recording.
Therefore, a rewritable phase-change medium and a recording method have been desired whereby recording can be carried out at a higher velocity.
However, a rewritable phase-change medium capable of recording up to a high linear velocity over 32-times velocity for CD or over 10-times velocity for RW-DVD, has not yet been realized.
A first reason for why such a rewritable phase-change medium can not be realized, is that it is difficult to simultaneously satisfy the archival stability of amorphous marks and erasing in a short time by high speed crystallization of amorphous marks.
The present inventors have already found a recording layer material comprising Sb as the main component. If such a material is used, overwriting at a recording linear speed of about 50 m/s will be possible.
A second reason for why CD-RW or RW-DVD overwritable at a high data transfer rate of at least 40 m/s has not yet been realized in spite of the fact that several phase change recording materials overwritable at such a high linear velocity have been proposed, is that the known conventional recording pulse strategy (pulse division method) has its limits.
Namely, in CD-RW standards Orange Book, Part 3, a recording pulse strategy as shown in FIG. 1, is specified. In a currently practically used recording device, IC for generating such a recording pulse strategy is employed. Accordingly, with such a device, it is obliged to carry out recording in a wide range of linear velocity ranging from 1-time velocity to 8- to 10-times velocity or to 8- to 32-times velocity by such a recording pulse strategy or by a recording pulse strategy having certain changes made thereto.
Also in standards for DVD-RW or DVD+RW as standards for rewritable DVD, a similar recording strategy is specified. A characteristic of such a recording strategy is that an amorphous mark having a nT mark length (T is the reference clock period) is divided into n-1 recording pulses and cooling pulses (off-pulses) for recording. Therefore, in such a recording strategy, an average repeating period for a pair of a recording pulse and a cooling pulse is made to be about 1T.
FIG. 1(a) represents an example of the mark length-modulation method used for a CD format and shows data signals having time lengths of from 3T to 11T, and FIGS. 1(b) and 1(c) show the practical recording laser powers generated on the basis of such data signals. Hereinafter, the recording pulse strategy wherein on and off of recording pulses are repeated based generally on the reference clock period T(100), as shown in FIG. 1(b), will be referred to as 1T strategy, and the recording pulse strategy wherein on and off of the recording pulses are repeated generally in periods 2T i.e. twice the reference clock period, will be referred to as 2T strategy. Pw represents a writing power to form an amorphous mark by melting and quenching the recording layer, Pe represents an erasing power to erase an amorphous mark by crystallization, and usually, a bias power Pb is substantially the same as a retrieving power Pr of a retrieving laser beam. Writing power (Pw) irradiation sections will be referred to as recording pulses, and bias power irradiation sections will be referred to as cooling pulses (the “cooling pulses” may sometimes be referred to as off-pulses). In the case of EFM+ modulation, data signals having time lengths of 14T are added to the above-mentioned data signals having time lengths of from 3 to 11T.
Here, in the above-mentioned recording strategies, a repeating period for a recording pulse and an off-pulse is basically constant as a reference clock period T or as a 2-times period 2T. The reference clock period T is made to have a high frequency in proportion to the linear velocity in high linear velocity recording.
At a reference velocity of 1-time velocity for CD, T=231 nsec, but at 40-times velocity, T=5.8 nsec, and at 48-times velocity, T=4.7 nsec. Accordingly, even in a case where the 2T strategy shown in FIG. 1(c) is used in high linear velocity recording at at least 40-times velocity, the time widths of divided recording pulses and off-pulses will be at most about 6 nsec by the above-mentioned change for high clock frequency corresponding to the high velocity recording.
On the other hand, at a reference velocity of 1-time velocity for DVD, T=38.2 nsec, but at 10-times velocity, T=3.82 nsec, at 12-times velocity, T=3.2 nsec, and at 16-times velocity, T=2.4 nsec. Accordingly, in high linear velocity recording at at least 10-times velocity, even if the 2T strategy as shown in FIG. 2(c) is used, the time widths of divided recording pulses and off-pulses will be at most about 4 nsec by the above-mentioned change for high frequency corresponding to such high velocity recording.
Whereas, by irradiation with a laser beam having a usual writing power, it takes from 1 to 2 nsec in rising or falling. Accordingly, at such a high frequency, the rise time or the fall time can not be neglected, and the lengths of recording pulse sections and the lengths of off-pulse sections will further substantially be shortened and will be substantially less than 5 nsec (in the case of CD-RW) or less than 3 nsec (in the case of RW-DVD). In such a case, heating for recording pulses tends to be inadequate, and the required writing power will be remarkably high. On the other hand, cooling for the off-pulse sections also tends to be inadequate, whereby a cooling rate required for the change into an amorphous state tends to be hardly obtainable. Further, for the high linear velocity recording, it is common to employ a material having a high erasing speed i.e. a high crystallization speed for the recording layer for CD-RW or RW-DVD. Accordingly, deficiency in the cooling rate for the above-mentioned off-pulse sections, tends to lead to recrystallization of the once-melted region. This tendency tends to be remarkable as recording becomes high linear velocity and high data transfer rate (high density).
Such a problem tends to be most remarkable with a phase change type rewritable optical recording medium (which may sometimes be referred to as “a rewritable phase change medium” in the present invention). However, in a case where mark length-modulation recording is carried out by controlling both the heating process and the cooling process by using divided recording pulses as shown in FIG. 1, as the clock frequency becomes high, the problems of delay in thermal response due to a heat capacity of the recording layer and the limit in response time of the laser diode optical output, tend to be increasingly remarkable in optical recording in general.
The present inventors have already realized overwriting recording at 20 times velocity or more for CD and at 5-times velocity or more for DVD, by the 2-T strategy wherein the repeating period of a recording pulse and an off pulse is set to be 2T (Proceedings of PCOS2000, The Society of Phase Change Recording, Nov. 3, 2000, Nov. 30-Dec. 1, 2000, p. 52-55, Proc. SPIE, The International Society for Optical Engineering, 2002, No. 4090, p. 135-143, Proc, SPIE. The International Society for Optical Engineering, 2000, No. 4342, p. 76-87, JP-A-2001-331936).
However, it has been found that even if such a division method of 2T base as reported in the above references, is employed, it is necessary, as mentioned above, to employ a material having a high crystallization speed for high linear velocity recording at a level of at least 32-times velocity for CD or at a level of at least 12-times velocity for DVD, while, if such a material is employed, the recrystallization phenomenon will be more serious due to deficiency of the cooling rate.
Such a problem is not limited to a phase change type rewritable optical recording medium, but is a common problem in a case where recording is carried out at a high data transfer rate (a high reference clock period, a high linear velocity) with a medium to which a recording method of controlling both the heating and cooling processes, is applied by means of divided recording pulses.
In such a situation as described above where the reference clock frequency becomes high, and the reference clock period T becomes generally less than 5 nsec, it is conceivable to reduce the recording pulse dividing number, as a natural extension. In reality, several division methods with n/3 have been disclosed (JP-A-2003-30836, WO02/089121).